1. Field of the Invention
The present invention relates to a motor-driven steering controller and more particularly to a motor-driven steering controller which provides assist steering torque depending on difference in braking force between right and left wheels and an anti-skid controller including such a motor-driven steering controller.
2. Background Art
When braking sudden enough to cause anti-skid control is executed on a road where the frictional coefficient differs from right to left, i.e., so-called μ split road, the resulting braking force differs from right side to left side of a vehicle. The difference in braking force between the right and left sides causes generation of yaw moment that causes the vehicle to be deflected toward the higher road frictional coefficient side. On the μ slit surface of an actual road, snow or ice can be left on the edge of the road to reduce the frictional coefficient thereof while the asphalt on the central part of the road is dried or wet to have a raised frictional coefficient. When a vehicle having its right and left wheels positioned on the edge of the road and on the central part of the road, respectively, is suddenly braked, the resulting braking force is larger on the central part of the road, which has a large frictional coefficient, than on the edge of the road, which has a smaller frictional coefficient. As a result, yaw moment is developed on the vehicle to cause the vehicle to be deflected toward the central part of the road.
In order to reduce yaw moment generated due to the difference in braking force between right and left, the operator needs to steer in a direction opposite the deflecting direction of the vehicle. This steering is known as counter-steering. However, this counter-steering requires some deal of skill.
On the other hand, anti-skid control is normally arranged such that the braking force on the front wheels is subjected to yaw moment inhibition control while the braking force on the rear wheels is subjected to select low control to inhibit sudden yaw moment caused by the difference in braking force between right and left when sudden braking is executed on μ split road. The yaw moment inhibition control is adapted to raise the time gradient of braking force on the wheel on high frictional coefficient side (i.e., reduce the gradient of rise of braking force with respect to time) when the road is judged to be μ split road, to thereby inhibit the generation of sudden yaw moment. The select low control is adapted to control the braking force on high frictional coefficient side according to the braking force on low frictional coefficient side. Both the yaw moment inhibition control and the select low control reduce the braking force on high frictional coefficient side, giving a long braking distance.
In this respect, JP-A-8-183470 proposes a power steering device which reduces steering torque necessary for steering according to the difference in braking forces between right and left wheels to facilitate a counter-steering operation. It is specifically described that, when the difference in braking forces between the right and left wheels is sensed to be large while operating the anti-skid controller, solenoid valves are controlled to control the hydraulic pressure to be supplied into a reaction variable mechanism in control valves so that the steering reaction to be imposed on the operation of the steering wheel by the reaction variable mechanism is reduced.
JP-A-11-129,927 proposes the running wheel control structure of a motor-driven steering vehicle provided with a torque steer inhibition controlling unit. In this, a problem is taken into account that when a vehicle having a right and left wheel control system on board is controlled such that driving forces (or braking forces) which are not equal from right to left are acted on the wheels to be steered (front wheels in ordinary vehicle), moment developed around the king pin by driving/braking force differs from the right wheel to the left wheel when an ordinary suspension is used, causing a steering kickback, loss of control over a steering wheel due to the turning of the wheels to be steered.
The above cited official gazette describes an arrangement of the device as follows. That is, when there occurs difference in control value between the right and left wheel while the driving/braking force control is separately executed over the right and left wheels, torque steer inhibition control signal is outputted to the motor-driven power steering device to cancel moment (torque steer) developed around the king pin due to the difference in driving force/braking force between the right and left wheels, making it possible to execute torque steer inhibition control in addition to ordinary assist control by the motor-driven power steering device.